1. Field of the Invention
The present invention relates to a transmission method and a transmitter according to the ultra wide band (UWB) system and a reception method and a receiver according to the UWB system.
2. Description of Related Art
Particular attention has been paid to the UWB system as one of wireless transmission systems. The UWB system realizes transmission using a very wide transmission band of, for example, several gigahertzes and using very short pulses.
FIG. 13 shows a configuration example of a conventional UWB transceiver. An antenna 11 is connected to an antenna changer 13 via a band-pass filter 12. The antenna changer 13 is connected to reception-related circuits and transmission-related circuits. The antenna changer 13 functions as a selection switch to operate in interlock with transmission and reception timings. The band-pass filter 12 passes signals of transmission band widths of several gigahertzes such as 4 GHz to 9 GHz used for the system.
The reception-related circuits connected to the antenna changer 13 include a low noise amplifier 14, 2-system multipliers 15I and 15Q, low pass filters 16I and 16Q, and analog-digital converters 17I and 17Q. The low noise amplifier 14 amplifies an output from the antenna changer 13 for reception. The multipliers 15I and 15Q multiply an output from the low noise amplifier 14 by outputs from pulse generators 25I and 25Q. The low pass filters 16I and 16Q eliminate high frequency components from outputs from the multipliers 15I and 15Q. The analog-digital converters 17I and 17Q sample outputs from the low pass filters 16I and 16Q.
Output pulses from the pulse generator 25I and 25Q are phase-shifted from each other by the specified amount. The analog-digital converter 17I samples I-channel transmission data. The analog-digital converter 17Q samples Q-channel transmission data. Received data for each channel is supplied to the baseband circuit 30 for reception processing. In this example, received data for the I channel is used as is. Received data for the Q channel is used as an error signal.
As transmission-related circuits, the multiplier 26 is supplied with transmission data output from the baseband circuit 30. The transmission data is multiplied by an output from the pulse generator 25I. The transmission data output from the baseband circuit 30 is modulated, e.g., as an NRZ (Non Return to Zero) signal. The multiplier 26 multiplies the transmission data by an output from the pulse generator 25I to generate a bi-phase modulated pulse. This becomes a signal modulated by the so-called BPSK (Binary Phase Shift Keying) system. In order to allow the pulse generator 25I to generate pulses, there is provided a Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO, hereafter simply referred to as an oscillator) 21 to control oscillation frequencies of the oscillator 21 based on an error signal acquired from received data for the Q channel.
An oscillation signal from the oscillator 21 is supplied to a PLL (phase locked loop) circuit 22. A voltage control oscillator 23 constitutes a loop for the PLL circuit 11. An oscillated output from the voltage control oscillator 23 is supplied to the pulse generator 25I to generate a pulse synchronized to the oscillated output from the oscillator 23. A phase shifter 24 supplies a pulse generator 25Q with an output from the oscillator 23 by delaying a specified cyclic phase. This makes it possible to generate a short wavelength pulse synchronized with the oscillated output from the oscillator 23 at a timing delayed from an output pulse of the pulse generator 25I.
A multiplier 26 multiplies an output pulse from the pulse generator 25Q by the transmission data to use the multiplication output as a transmission signal. The transmission signal output from the multiplier 26 is supplied to a power amplifier 27 and is amplified there for transmission. The amplified output is supplied to the band-pass filter 12 via the antenna changer 13. The band-pass filter 12 limits the band to pass only signals for the transmission band. The transmission signal is then transmitted from the antenna 11.
FIG. 14 shows a process example in the baseband circuit 30. A despreader circuit 31 is supplied with received data for the I and Q channels for a despread process, i.e., the reverse of transmission despreading. The despread received data for the I channel is supplied to a data demodulation circuit 32 for demodulation. The received data is then supplied to a CRC circuit 33 for error detection and correction. The processed received data is supplied to a UWB communication management and processing section 34 for processing in layers specified in this communication system.
A loop filter 35 extracts an error component from the Q channel's received data despread in the despreader circuit 31. The error component is supplied as a control signal to the oscillator 21 in FIG. 13.
FIG. 15 shows an example of frequency spectrum for transmission signals. The example in FIG. 15 uses a band of approximately 10 GHz. FIG. 16 exemplifies a time waveform of transmitted signals. The UWB system transmits a very short pulse of one nanosecond or less. It is known that such a short wavelength pulse has a very wide bandwidth of at least several gigahertzes on a frequency axis. Accordingly, there is provided the frequency spectrum as shown in FIG. 15.
Transmission signals may be available not only in the mono-cycle (one cycle) pulse waveform as shown in FIG. 16, but also in a 2-cycle or 3-cycle pulse waveform. FIG. 17 shows a time waveform according to the 2-cycle pulse (bicycle pulse). The 2-cycle pulse can increase a transmission power compared to the 1-cycle pulse.
When a signal is transmitted in this manner and is received, the received signal is held for synchronization as follows. For example, a pulse is delayed from the I-channel signal for specified amount 1. This pulse is used as a template waveform for the Q channel to find a value of correlation between the received signal and the template waveform. The oscillation phase of the oscillator is controlled based on the correlation value. FIG. 18 exemplifies a cross-correlation waveform during transmission of the 2-cycle pulse in FIG. 17. When the oscillation phase is controlled based on the correlation values as shown in FIG. 18, it becomes possible to perform reception processing in precise synchronization with received data.
Non-patent document 1 outlines the UWB system.
[Non-Patent Document 1]
Nikkei Electronics, 11 Mar. 2002, pp. 55–66.
Presently, the UWB system is subject to the FCC (Federal Communications Commission) specifications in the U.S. The FCC specifications include radiation intensities for indoor and outdoor frequency bands. For example, the transceiver in FIG. 13 needs to be configured so that the band-pass filter 12 connected to the antenna 11 can provide the spectrum compliant with the specifications. If the band-pass filter performs a process needed for this purpose, however, a filter's group delay greatly oscillates the pulse waveform, causing an inter-pulse interference. When an inter-pulse interference exists, it is necessary to increase a time interval between pulses, decreasing a chip rate. The interference can be reduced if it is possible to increase the interval between pulses by maintaining the chip rate. This has been difficult with the conventional processing.
The conventional UWB system basically uses all provided transmission bands, e.g., bands permitted for use by FCC. The gigahertz bands to be used for the UWB system include bands already used for the other systems. It is necessary to limit the transmission power for the already used bands. However, the band-pass filter needs to limit transmission signal bands in order to limit only part of the bands. As mentioned above, if the band-pass filter limits bands, a filter's group delay deforms the pulse waveform and decrease the transmission efficiency.